A natural consequence of using a linear-phase Finite Impulse Response (FIR) lowpass filter for antialiasing during the resampling process is that a time delay is introduced due to the filter's constant group delay across all frequencies.
In practice, this delay is small (of the order of 1 or 2 milliseconds).
The actual delay (in samples) is given by the following formula:
where:
D = Delay (in samples)
N = length of FIR
L = decimation factor (denominator in conversion ratio)
To correct the timing due to the delay, ReSampler discards (ie does not write to the ouptut file) samples prior to the Dth output sample.
This behavior appears to be the accepted practice for sample rate converters, but can nevertheless be disabled using the --noDelayTrim option.
The --noDelayTrim option has no effect when used in conjunction with Minimum Phase, since a minimum-phase FIR should not introduce delay.
Note: In a realtime system, removing the delay would not be possible because it would require the ability to go back in time!
The following illustrates the group delay introduced when a 440Hz square wave is resampled from 96kHz to 44.1kHz:
above: 440Hz square wave at 96kHz sampling rate
above: 440Hz Square wave, resampled from 96kHz to 44.1kHz with no delay correction.
Grey area indicates delay due to linear phase FIR, and this grey region is trimmed off unless --noDelayTrim option is used